The proposed study will examine sibling socialization effects and the development of adolescent-onset, non-aggressive behavior problems. The goal of the investigation is to understand under what conditions the sibling relationship promotes the development of deviant behavior. The study is the first time point in a longitudinal analysis of connections between sibling relationships, peer experiences, and adolescent-onset delinquency. Hypotheses, based on biosocial and social learning theories, will be tested in relation to each aim. The three aims of the study are: (1) To apply a unique design to examine the nature of childrens sibling relationships during middle childhood; (2) To examine twin and friend similarity in behavior problems, cognitive abilities, personality, and peer experiences; (3) To test a model of twin/sibling relationship effects on connections between peer experiences and behavior problems. The novel design includes 342 pairs of children (and their parents) between 8 and 11 years old in six same-age dyads: 57 monozygotic twin pairs, 57 same-sex dizygotic twin pairs, 57 opposite-sex dizygotic twin pairs, 57 same-sex, same-aged, unrelated sibling pairs, 57 opposite-sex, same-aged, unrelated pairs and 57 friend-friend pairs. Same-age, unrelated siblings raised together since infancy replicate the twin situation but without genetic relatedness. In addition, a friend of each twin in the same-sex twin pairs will be interviewed over the telephone. A variety of methods will be used to assess the relevant constructs (e.g., standardized assessments, parent and child reports, observations of social interaction, and telephone interviews with parents, siblings, and friends). Levels of mutual respect, competition, cooperation, warmth, conflict, and temporal involvement in the sibling relationship will be assessed. Multiple aspects of the childrens friendships and peer groups will be examined, as well as the quality of the parent-child relationships. The role of childrens and friends personal characteristics and interests will be also explored. Outcome measures will include assessments of childrens levels of aggression, delinquency, loneliness, depression, and anxiety, as well as childrens personality characteristics and cognitive abilities.